1. Field of the Invention
This invention is related in general to the field of interferometry and, in particular, to a method and apparatus for optimizing the focal-point calibration of an interferometric microscope.
2. Description of the Related Art
Microscope objectives are commonly used in interferometric devices for focusing a beam of light on a sample surface and a reference surface to produce interference fringes representative of the optical path difference (OPD) between the test and reference path. As illustrated in simple schematic form in FIG. 1, a typical interferometric microscope 10 consists of a microscope objective focused on a test surface S and incorporating an interferometer. The interferometer, shown in Linnik configuration for illustration, comprises a beam splitter 12 and a reference mirror R such that the light beam W directed to the sample surface S is split and also directed to the reference mirror. As is well understood by those skilled in the art, the light beams reflected from the reference mirror R and the test surface S (the reference and test beams, respectively) are combined to produce interference fringes as a result of the optical path difference between the reference mirror and the test surface S. The light is typically passed back through the interferometric microscope objective 10 and appropriate imaging optics 14 toward an imaging array 16 positioned in a camera in coaxial alignment with the objective. The imaging array usually consists of individual charge-coupled-device (CCD) cells or other sensing apparatus adapted to record a two-dimensional array of signals corresponding to interference effects produced by the interferometer as a result of light reflected at individual x-y coordinates or pixels in the surface S and received at corresponding individual cells in the array. Appropriate electronic hardware (not shown) is also provided to process the signals generated by each cell and transmit them to a computer 17 for further processing. Thus, an interference-fringe map is generated by detecting the intensity of the light signal received in each cell of the array. The map may be displayed on a monitor 19 connected to the processing unit 17. Additional information about the test surface S can be gained by varying the OPD between the reference and test paths with a scanning device (not shown) shortening or lengthening either the reference path or the test path.
The present invention is directed at improving the focus calibration of the reference mirror R in such an interferometric microscope. As shown in FIG. 1, the Linnik configuration of a microscope objective includes a white light source 18 and imaging optics 20 providing an illumination beam W to the system through a beam splitter or equivalent device 22. The illumination beam is then focused, ideally, in the entrance pupils E and Exe2x80x2 of the reference and test imaging optics 24 and 26, respectively. This is known as Kohler illumination, and produces approximately uniform illumination on the sample surface S and the reference surface R. By way of calibration, the reference mirror R is set at the focal point of the imaging optics 26 through a process based on a visual determination of best focus produced by axially shifting the reference mirror with respect to its imaging optics 26, as one skilled in the art would readily understand. One common way of determining best focus is by imaging a variable aperture, known as a field stop, onto the reference mirror such that the image of the edges of the aperture, upon reflection from the reference mirror, is sharply in focus at the imaging array 16 when the reference mirror is in focus. Once the optimal distance x between the lens 26 and the mirror R is found my manual manipulation, the position of the mirror within the microscope 10 may be fixed at the factory for a particular instrument. However, due to practical limitations on stability, it often must be left adjustable and set by the user during setup procedures.
Such manual setting of the reference-mirror focus suffers from two distinct limitations. First, it is based on an operator""s visual observation of best focus, which is necessarily approximate because of the inability of the human eye to distinguish image variations produced by very small focal shifts. Thus, it is limited at best to a precision equal to the depth-of-focus of the objective (often considered to be +/xe2x88x920.25 wavelengths of defocus in the converging wavefront). Second, the approach is inherently subjective, leading to different results from different operators.
Moreover, as interference microscopes have become standard quality-control tools in production environments, such as for testing the topography of magnetic heads, greater measurement precision and repeatability are required. Such interference microscopes are now capable of making measurements with sub-nanometer precision. Accordingly, the precise in-focus position of the reference mirror has become more and more critical. As is well known in the art, the depth of focus (DOF) of an objective is inversely proportional to the square of its numerical aperture. Thus, the depth of focus decreases rapidly with the objective""s magnification. For example, while a Nikon(copyright) 5xc3x97/0.13NA objective has a depth of focus of about 30 microns, a Nikon(copyright) 100xc3x97/0.95NA objective has a DOF smaller than one micron. Therefore, minute shifts in the position of the reference surface R with respect to its imaging optics 26 are sufficient to cause the reference surface to be out of focus and produce incorrect profile measurements. Accordingly, it is now necessary to obtain extremely tight levels of control over the defocusing effects of the reference mirror and it has become desirable to periodically and routinely recalibrate interferometric microscopes.
A significant improvement toward these ends was achieved with the development of athermalized interference objectives to suppress the effects of environmental changes. As detailed in U.S. Pat. No. 5,978,086, incorporated herein by reference, in these objectives the reference mirror is held at the optimal focus location over a range of temperatures by adding structural components that offset the effects of temperature variations. Each component is coupled sequentially, such that the thermal response of the objective assembly is substantially the linear combination of the response of each component and is designed to cause a shift in the opposite direction to the shift produced by a temperature change in the unmodified device. The thermal characteristics and dimensions of the components are chosen empirically to minimize the shift of the reference mirror with respect to its in-focus position in the interferometric objective as a function of temperature.
In spite of this improvement, defocusing effects result from vibrations, wear, and other environmental forces that affect the reference mirror in addition to temperature changes. Thus, it remains desirable, and for some applications it has become necessary, to assure the current in-focus position of the reference surface of interferometric microscopes. This invention is directed at providing a solution to this problem. Furthermore, this invention readily allows focus of the reference mirror with a level of precision that cannot be obtained with a visual focus technique.
One primary object of this invention is a method for setting the in-focus position of the reference surface of an interferometric microscope objective that is not operator-dependent.
Another object of the invention is a method that is suitable for automated implementation.
Still another objective is a method and corresponding implementing apparatus that are suitable for periodic calibration of an interferometric microscope objective while in service.
Another goal of the invention is a method and apparatus that are suitable for incorporation within existing instruments.
A final object is a procedure that can be implemented easily and economically according to the above stated criteria.
Therefore, according to these and other objects, the present invention consists of incorporating a laser interfero-meter into an interference microscope to precisely determine the in-focus position of the microscope objectives reference mirror. A collimated laser beam is introduced into the microscope system and split into two beams directed toward a special calibration reference surface and the interference objective. The light reflected from the special reference surface is returned to the camera. The light into the interference objective is focused onto the reference mirror and returned to the camera. When the reference mirror is in focus, the returned beam is collimated; if the mirror is on either side of focus, the beam is either converging or diverging. Accordingly, the interferogram produced at the camera reflects the in-focus to out-of-focus condition of the reference mirror. For the purpose of calibration, the objective""s reference path length is scanned to shift the interference fringes produced by the interference of the beam reflected by the reference mirror with the light reflected by the calibration reference surface. Equivalently, the path length of the beam reflected by the special reflectance surface can be scanned for the purpose of calibration. The curvature of the wavefront returned from the reference mirror is determined electronically by analyzing the interference fringes produced with the beam returned from the calibration reference surface. By minimizing the curvature of the reference-mirror wavefront as the mirror is translated along the optical path, the reference mirror can be focused with an accuracy greater than possible by visual observation. Furthermore, by automating the focusing system with a precise translation mechanism driven by closed-loop control, operator-to-operator variations are completely eliminated.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose but one of the various ways in which the invention may be practiced.